An emulsion is a fluidic state which exists when a first fluid is dispersed in a second fluid that is typically immiscible or substantially immiscible with the first fluid. Examples of common emulsions are oil in water and water in oil emulsions. Multiple emulsions are emulsions that are formed with more than two fluids, or two or more fluids arranged in a more complex manner than a typical two-fluid emulsion. For example, a multiple emulsion may be oil-in-water-in-oil (“o/w/o”), or water-in-oil-in-water (“w/o/w”). Multiple emulsions are of particular interest because of current and potential applications in fields such as pharmaceutical delivery, paints and coatings, food and beverage, chemical separations, and health and beauty aids.
Typically, multiple emulsions of a droplet inside another droplet are made using a two-stage emulsification technique, such as by applying shear forces through mixing to reduce the size of droplets formed during the emulsification process. Other methods such as membrane emulsification techniques using, for example, a porous glass membrane, have also been used to produce water-in-oil-in-water emulsions. Microfluidic techniques have also been used to produce droplets inside of droplets using a procedure including two or more steps. For example, see International Patent Application No. PCT/US2004/010903, filed Apr. 9, 2004, entitled “Formation and Control of Fluidic Species,” by Link, et al., published as WO 2004/091763 on Oct. 28, 2004; or International Patent Application No. PCT/US03/20542, filed Jun. 30, 2003, entitled “Method and Apparatus for Fluid Dispersion,” by Stone, et al., published as WO 2004/002627 on Jan. 8, 2004, each of which is incorporated herein by reference. See also Anna, et al., “Formation of Dispersions using ‘Flow Focusing’ in Microchannels,” Appl. Phys. Lett., 82:364 (2003) and Okushima, et al., “Controlled Production of Monodispersed Emulsions by Two-Step proplet Breakup in Microfluidic Devices,” Langmuir 20:9905-9908 (2004). In some of these examples, a T-shaped junction in a microfluidic device is used to first form an aqueous droplet in an oil phase, which is then carried downstream to another T-junction where the aqueous droplet contained in the oil phase is introduced into another aqueous phase. In another technique, co-axial jets can be used to produce coated droplets, but these coated droplets must be re-emulsified into the continuous phase in order to form a multiple emulsion. See Loscertales et al., “Micro/Nano Encapsulation via Electrified Coaxial Liquid Jets,” Science 295:1695 (2002).
Multiple emulsions and the products that can be made from them can be used to produce a variety of products useful in the food, coatings, cosmetic, chemical, or pharmaceutical industries, for example. Methods for producing multiple emulsions providing consistent droplet sizes, consistent droplet counts, consistent coating thicknesses, and/or improved control would make commercial implementation of these products more viable.